In an optical telecommunications network data is transmitted in the form of pulses of light, in which a pulse of light represents a logic state "1" and the absence of a pulse, or a pulse of different magnitude, represents a logic state "0". In low data rate systems it is known to produce the pulses of light by driving a solid state laser using a signal which is related to the data to be transmitted. Lasers, however, can "chirp"; that is they no longer produce pulses of light of a single wavelength. This spreading of the wavelength of light results in dispersion of the pulses as they travel along optical fibres which can ultimately limit the operating frequency and/or range of the optical network.
To overcome the problems associated with laser "chirp" it is known, when operating at high data rate, to run the laser continuously and externally modulate the continuous light output using an optical modulator. The optical modulator applies variable attenuation to light passing through it, the amount of attenuation being dependent upon an electrical signal applied to a control input. One example of such a modulator is a lithium niobate Mach Zehnder modulator which has an optical transfer response (that is optical attenuation versus the voltage applied to the control input) which is approximately sinusoidal in shape. In an optical telecommunications network in which digital data is being transmitted it is often required that the data fully modulates the light output, that is light should pass substantially unattenuated for a logic state "1" and be completely attenuated for a logic state "0" or vice versa. To achieve such modulation requires the modulator to be operated at voltages which correspond to the maxima and minima in the optical transfer response.
In practice whilst the shape of the modulator's transfer response may be known its relative position along the voltage axis drifts resulting in distortion of the transmitted signal. As described in U.S. patent Ser. No. 5,400,417 such drift can be compensated for by applying a DC bias voltage v.sub.bias to a second input of the modulator to maintain the modulator's transfer response at the desired voltage position. Control circuitry is provided which monitors the optical output of the modulator to determine by how much the modulator's transfer response has drifted and the DC bias voltage is adjusted accordingly to maintain the modulator's transfer response at the desired voltage position. Whilst such an arrangement is found to track any drift in the modulator's response, a problem can arise if the transfer response drifts by an amount which would require a bias voltage which exceeds the maximum bias voltage available as set by the power supply of the control circuitry or by constraints in the modulator.
When the modulator's response drifts by an amount such that the bias voltage required for correct operation exceeds the available bias voltage range it is necessary to reset the system.
For an optical modulator which has a transfer response which is cyclic it has been proposed in U.S. Pat. No. 5,003,624 to reset the bias voltage by an amount corresponding to one cycle of the response. In this way the modulator is reset to a corresponding portion of its transfer response. However with such a system a loss of the transmitted data will occur during the period that the voltage is adjusted. In the case of some transmission systems, for example the analogue transmission of television signals, this may be acceptable. In digital telecommunication networks however, such as those operating using a synchronous digital hierarchy (SDH) format, there are strict constraints on performance. In particular in high bit rate systems, such as would typically use external optical modulators of this type, resetting the system could corrupt unacceptably large quantities of data.
The present invention addresses the technical problem of resetting an optical modulator without causing appreciable disruption to the data and in particular concerns the problem of resetting a Mach Zehnder optical modulator which is used in an optical telecommunications network operating with a synchronous digital hierarchy (SDH) format.